Aircraft pressure control system



March 29, 1949. H, T, SPARROW 2,465,759

AIRCRAFT PRESSURE CONTROL SYSTEM ro CONT/MLS 7/ OUHET j/ mim/ifm? /7 Gttorneg March 29,

Filed June 22, 1944 H. T. sPARRow AIRCRAFT PRESSURE CONTROL SYSTEM 5 Sheets-Sheet 2 7W-gull 4/0 March 29, v1949.

H. SPARROW AIRCRAFT PRESSURE CONTROL SYSTEM 3 Sheets-Sheet 3 Filed June 22', 1944 Patented Mar. 29, 1949 AIRCRAFT PRESSURE CONTROL SYSTEM Hubert '.li. Sparrow, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application .lune 22, 1944, Serial No. 541,520

28 Claims.

l The present invention is concerned with aircraft pressure control systems and more particularly with a system for controlling means for supplying air under pressure to both the engine and the cabin of an aircraft propelled by internal combustion engines and having a cabin adapted to be maintained normally at a higher pressure than the surrounding atmosphere,

It is common practice in connection with interna] combustion engine propelled aircraft operating at higher altitudes to employ some form of compressor, commonly known as a supercharger, for increasing the pressure of the air forming a part of the fuel mixture of the supply to the internal combustion engine. One common form of said supercharger has been the turbine supercharger in which the turbine is operated by exhaust gases from the engine. In planes operating at relatively high altitudes, it is desirable not only to increase the pressure of the air supplied to the engine but also to increase the pressure within the cabin. It has previously been proposed to do this by providing a separate compressor usually driven directly from the air-craft engine. The present invention contemplates the use of the lsame supercharger for both supercharging the air supplied to the internal combustion engine and also maintaining the proper pressure within the cabin.

It is broadly an object of the present invention to provide in connection with an aircraft propelled by an. internal combustion engine and having a cabin normally maintained at a higher pressure than the surrounding atmosphere, a motor operated compressor for supplying air to both the engine and the cabin with means for controlling the motor operating the compressor in accordance With conditions affecting both engine operation and the need for air in the cabin.

A further object of the present invention is to provide such a system in which the pressure of the air is normally regulated so as to assume a value selected for optimum engine operation with means to insure that the pressure selected will never be too low to insure the maintenance of a proper pressure within the cabin.

A further object of the present invention is to provide an arrangement in which the means for limiting the extent of adjustment of the pressure is responsive to cabin pressure.

An alternative object of the invention is to provide such an arrangement in which the limiting means is responsive to atmospheric pressure.

Still a further object of the present invention 2 is to provide a novel means for limiting the effect of the pressure adjusting means in accordance with cabin pressure.

A still further object of the present invention is to provide a system of the type considered above in which the compressor is operated by a motor driven by the exhaust gases from the engine and in which the means for controlling the motor is compensated whenever the throttle is moved so as to compensate for a change in volume of the exhaust gas.

A still further object of this invention is to provide in connection with an aircraft having a plurality of engines and compressors therefor, the compressors of certain engines being employed to increase the pressure of the cabin and others not being so employed, compensating means operated by the throttle of the engines whose compressors are to introduce the necessary compensation into the control systems for all of the compressors to maintain the power output of the engines at substantially the same value.

A further object of the present invention is to provide a novel means of controlling the flow of air between the compressor and the cabin in connection with a system of the type discussed above.

A still further object of the present invention is to provide a novel means for controlling cabin pressure in which the cabin pressure can be changed only at a timed rate.

A still further object of the invention is to provide a novel form of electrical control system for controlling the pressure within the cabin.

Other objects will be apparent from a consideration of the accompanying specification, claims, and drawings of which Figure 1 is a schematic representation of an aircraft pressure control system controlled in accordance with the present invention,

Figure 2 is a schematic representation of a modified form of my system, and

Figure 3 is a schematic representation of the electrical control portion of a still further modiication of my system.

Referring to the drawings, the system is illustrated in connection With ya four engine aircraft -in which the air supplied to the carburetor is compressed by a turbine operated supercharger, the turbine being driven by exhaust gases. In order :to facilitate a clearer understanding of the invention, the various elements have been shown schematically. In Figure 1, .the lcontrols for only engine number two are illustrated, this engine being one of the inboard engines. In the proposed embodiment of my invention, the compress-ors of engines number two and number three, the two inboard engines, are employed to supply air under pressure to :the cabin. The compressors of the outboard engines, engines number one and four, are not connected with the cabin and .are employed merely in lthe usual manner to supply supercharged air to the carburetors of their respective engines.

Referring now to Figure 1, engine number two is schematically indicated by the reference character I9. Air for supporting combustion in the engine is supplied by compressor II receiving air from the atmosphere through a pipe I2 connected with any suitable air intake and delivering the air under compression through a duct I3, an after cooler lil, a duct I5 in which is loca-ted a throttle i3, -a carburetor I1, a direct driven compressor I8, and an intake manifold I9 to the engine I0.

The after cooler I4 removes 4the heat of compression f-rom the air discharged by compressor II, when this is required, and for this purpose receives fresh air through the intake pipe 20, this air being passed in heat exchanging relation to the air in the induction system and delivered as waste through an -outlet pipe 2 I.

The throttle It is of conventional construction and `is operatively :connected to an operating handle 23.

The carburetor i1 functions in the usual manner to mix fuel delivered from 4a source (not shown) with the air which is supplied by compressor II to form a fuel mixture of the proper proportion. The pressure of this mixture is increased by the direct driven compressor IB which is driven through a shaft 25 directly by the engine I0. Direct driven compressor I8 is usually built into the intake manifold I9 but is shown separately for convenience of illustration.

The compressor I! is driven through a shaft 21 by a turbine 28, the compressor and turbine together being commonly referred to as a turbo supercharger. The turbine 28 is powered by exh-aust gases from .the engine, receiving the exhaust gases through an exhaust manifold 3B and a duct 3l and discharging the gases through an outlet pipe 32. A by-pass du-ct 33 is shown as leading off from |the duct 3I and a damper or waste gate 34 is provided in this duct 33 -to con- :trol the flow of exhaust gases through an `outlet duct 35.

The resistance to flow of the exhaust gases through the duct 33 will, of course, be less than rthe resistance through the turbine 28 and hence as the waste gate 361 is opened, `the gases will to aan increasing extent discharge through conduit 33 rather th-an :through the turbine 28. As the waste gate is progressively closed, on the other hand, more and more of the total volume of exhaust gases Will be diverted through the turbine to increase its speed and that of the compressor II so as to increase the compression ratio of the latter. The lposition of the waste gate 34 is thus seen to control the speed of the turbo supercharger and the pressure of the air supplied thereby to the engine.

The waste gate 34 is positioned by a reversible two-phase alternating current motor 35 having an armature 31 and two field windings 38 and 39 spaced 90 electrical degrees apart. The adjacent terminals of the two windingsl 33 and 39 are connected together and to ground as indicated at 40. lThe motor is shown connected through a gear train 4I to the waste gate 34.

- A transformer 44 is employed for energizing the motor and the various ycontrol equipment for the motor. The transformer 44 comprises a primary winding 45 and la plurality of secondary windings including windings 415 and 41. It is to be understood that the transformer 44 includes other secondary windings which for convenience of illustration have been shown in connection with the various electrical elements with which they are immediately associated and will be referred to at the time that these elements are described.

The winding 41 of transformer 4f! is employed to continuously energize 7the eld winding 39, the secondary 41 being connected to the field Winding 39 by a conductor 50 and by a conductor 5I, a 4condenser 52, and a conductor 53. The condenser 52 is of such value that at the frequency of the source to which lprimary winding 45 is connected, the condenser 52 and the winding 39 are resonant. As a result, the inductive effect of winding 39 is overcome so that the current through winding 39 is in 4phase with the voltage -across :secondary l1 instead of lagging the same by ninety electrical degrees as would otherwise be the case.

The other eld winding 38 is connected by conductors 56 and 51 to the output terminals 59 and Sil of an -amplier 62. The amplifier 62 is supplied with energy from secondary Winding 46 to which it is connected bv conductors 63 and 64.

The amplier is of any suitable type in which the output of the amplifier has a definite phase relationship to the input voltage. A typical amplier of this type is the one shown in the c0- pending application of Albert P. Upton, Serial No. 437,561, filed April 3, 1942, now Patent No. 2,423,534, issued July 8, 1947. The amplifier is provided with input terminals 65 and 51. When an input voltage of one phase relationship is supplied to input terminals 55 and 51, the current supplied to winding 38 leads by ninety electrical degrees, the current flowing through winding 39. Under these conditions, the motor 36 rotates in one direction. When the signal voltage applied to terminals 65 and 61 is one-hundred-eighty degrees out of phase with that previously considered, the current supplied by amplier 52 to winding 33 will lag by ninety degrees that flowing through Winding 39. Under these conditions, the motor 35 will rotate in the opposite direction. Thus, the motor 35 will rotate in one direction or the other, depending upon the phase of the signal voltage applied to terminals 65 and 61.

The signal voltage applied to terminals 65 and G1 is the output voltage of a resistance network including a plurality of series connected individual networks 59, 13, and 1 I The network 69 comprises a secondary winding 13 which may constitute a secondary winding of the transformer 44 or any other transformer energized from the same source of energy as primary winding 45. In order to identify the Winding 13 as a transformer secondary winding, the primary winding 45 is again shown in association with winding 13. Also constituting part of the network 59 are two potentiometers 'I4 and 15. The potentiometer 14 comprises a resistance element 'i3 and a contact arm 11 movable thereover. The contact arm 11 is operatively connected by a shaft 'i8 to the gear train 4I so that upon the motor 36 being operated to position the waste gate 34, the contact arm 11 is similarly positioned. The potentiometer 14 functions as a rebalancing potentiometer as will be more fully explained hereinafter. The potentiometer 15 conssts of a resistance element 80 and a slidable contact arm 8|. The resistance 80 is provided at one end with a conductive bar 82. It is to be understood that movements of the Contact arm 8| over this conductive bar do not affect the output voltage of the bridge 69 so that minor variations in the position of the slider 8| will not have any disturbing effect. Contact arm 3| is connected by a shaft 05 to a velocity controller 85. This velocity controller is driven through a shaft 81 by the turbine 28 so that the velocity controller is driven at the same speed as the turbine. The velocity controller may be of any conventional construction and functions to move the contact arm 8| whenever the speed of the turbine rises above a desired value. A typical type of velocity controller suitable for the purposes of the present invention is shown in my co-pending application, Serial No. 476,797, filed February 22, 1943.

The left-hand terminal of the resistor 1S is connected to intermediate tap 88 of secondary 13. The right-hand terminal of resistor l@ is connected by a conductor 90 to the right-hand terminal of secondary 13. The left-hand terminal of resistor 80 is connected by a conductor 9| to the left-hand terminal of secondary 13. The conductive bar 32 constituting the right-hand terminal of resistor 30 Ais connected by a conductor 92 to a tap 93 of secondary 13. In one particular embodiment of my invention, a secondary winding of 42 volts was employed for winding 13. In this particular embodiment, the voltage between the left-hand terminal and tap 93 was 24 volts, that between taps 93 and 83, 6 volts, and that between tap 88 and the right-hand terminal, 12 volts.

Network is in the form of a bridge comprising a secondary winding 95 preferably of the transformer 44, and two potentiometers 93 and 94. The potentiometer 95 consists of a resistance member 91 terminating at its left-hand end in a conductive bar 98 and a contact arm 99 which slides over the resistance 91 and a Contact bar 90. Contact arm 99 is positioned through a shaft |00 by an acceleration controller Elli. The acceleration controller |9| may be any suitable device which will impart a movement to shaft |00 causing clockwise movement of contact arm S9 whenever its driven member is accelerated beyond a predetermined amount. The acceleration controller is connected by shaft 81 to the turbine 23 so as to be responsive to the acceleration of the turbine. An acceleration controller suitable for the purposes of the present invention is shown in my aforementioned co-pending application.

The potentiometer 94 consists of a resistance member |03 and a slider arm |04. The slider arm |04 is pivotally connected at |05 to a suitable support. Slider arm |il4 is also connected at |00 to a link i 01 which in turn is connected to two opposed bellows members |08 and |09. The bellows member |08 is an evacuated bellows. The bellows member |09 is connected through a tube ||0 with the duct 2| leading from the outlet of the compressor. The bellows |09 is thus subject to the discharge pressure of the compressor, or in other words, the induction system pressure. The bellows EGB is provided for compensating for changes in atmospheric pressure. Since bellows |03 and |09 are subjected to the same atmospheric pressure, it is obvious that any changes in atmospheric pressure will cancel out. Thus, the position assumed by arm |04 is always a function of the induction system pressure.

The network 10 is in the form' of a simple Wheatstone bridge, the right-hand terminals of resistors 91 and |03 being connected to the righthand terminal of secondary 95 by conductors ||2 and H. Similarly, the left-hand terminals of resistors 91 and E03 are connected by conductors i ifi and i5 to the left-hand terminal of secondary 95. As with the case of the secondary winding i3, the secondary winding 95 constitutes an addtional winding of transformer 44.

The network 1| includes a secondary winding i i1, which again may be a secondary winding of the transformer [i4 or of a separate transformer energized from the same source of power as primary winding 45. Connected to the secondary H1 is a potentiometer H8. This potentiometer consists of a slider H9 which cooperates with a pair of resistors 20 and |2| connected by a conductive bar |22. The resistors |20 and |2| and the conductive bar |22 are connected in series to the secondary H1 by conductors |23, |24 and |25. Slidable contact arm H9 is pivoted at |21 and is connected to an actuating rod |28 which in turn is pivotally connected to a link |29. Link i2@ is pivotally mounted at |30 and is operatively connected to a bellows |3|. The bellows E35 is evacuated so as to be unaffected by temperature and is located so as to be influenced by atmospheric pressure, Thus, the position assumed by rod |28 and hence slider I9 is at all times dependent upon atmospheric pressure. The connecting means between bellows 3| and Contact arm H9 including the lever |29 and rod |28 are so disposed that a drop in atmospheric pressure causes movement of the slider |9 to the left.

Connected between the slidable contact arm C99 and the right-hand terminal of resistor |2| is a potentiometer |32 comprising a manually positionable slidable contact arm |33 which is in Contact with a resistor |34. Potentiometer |32 is employed as a pressure selecting potentiometer and is set to maintain the desired compressor discharge pressure. A knob |42 is secured to arm |33 to position the same. The knob |42 carries a pointer cooperating with suitable indicia |43 to indicate the manifold pressure selected, the manifold pressure being related to the induction system pressure.

The resistance network 1| not only comprises the secondary H1 and the potentiometer ||8 but also includes a further potentiometer |35. This potentiometer |35 comprises a resistor |36 and a slider 21. The potentiometer E35 is one of four potentiometers. These potentiometers have been given the numbers |35, |39, |40, and |4I, Each of these potentiometers is designed to be associated with the controls for a diiferent engine. The secondary ||1 and potentiometers IIB and |32 are common to four networks, each of which includes a different one of these potentiometers and each of which is associated with a composite network constituting a part of the control mechanism for one of the engines. The network 1|, which is the one being considered and which forms part of the control arrangement for engine No. 2, is unaffected by the positions of the sliders of the other potentiometers |39, |40 and 4|.

The mechanism which has been described so far is that which is concerned with the control of the waste gate 34 and hence of the turbine 28. As previously indicated, the present invention is concerned not only with the control of the turbine but also with the control of the airplane cabin pressure. As will be more apparent from the subsequent description, the system which has .amplier 62.

`schematically represented by the reference numeral 52. Connected with the main conduit AIil Ais a conduit |53 which leads from the discharge of the supercharger of engine No. 3.

While conduits |50, lei and |53 have been shown for convenience of illustration as of substantially the same size as the conduits i3 and l5, it is to be understood that this is only for convenience in illustration and that actually these conduits would be of considerably smaller size. Disposed in conduits i563 and i5@ are valves |55 and |56. These valves are provided with manual operators |51 and |58, respectively. Suitable indicia may be disposed adjacent the operators H51 and |58 to indicate the open and closed positions of the valves. Normally, these valves are both open but may be closed when it becomes necessary to dispense with the operation of one of the engines.

Connected with the airplane cabin is a discharge conduit itil. This conduit is provided with an adjustable valve ISI provided with an operating handle |62. This valve is normally maintained in any desired adjusted position.

Disposed in the supply conduit icl is a Valve |64 of any suitable type. The valve lee is adapted to be driven by a motor i through a gear train schematically indicated at l. The motor |66 is of the two-phase type having a rotor |68 and two windings |69 and li which are connected together and to ground at iii. The relative direction of rotation of motor li and hence of the movement of valve tibi depends upon the phase relation between the currents flowing through windings |69 and |10.

A transformer VES is employed for energizing the motor |66 and the control apparatus therefor. The transformer H3 consists of a pair of secondary windings lii and H5 and a primary winding |16 connected to any suitable source of power such as the source to which primary winding 45 of transformer it is connected.

The secondary winding ll5 is provided for the purpose of energizing winding les. One terminal of this secondary winding is connected to one terminal of motor field winding ite by the conductcr ll'S. The other terminal of secondary winding H5 is connected to motor field winding |69 by conductor H9, condenser |83, and conductor |8|. The condenser i550 is ci such value that at the frequency of .the supply current, the winding ieg and condenser il are resonant. Consequently, the current through winding W9 is clisplaced in phase approximately 9G" from the phase position it would assume ii condenser i8!) were not present.

The other secondary winding VM oi transformer 13 is employed to energize an amplifier |85, being connected to the input terminals |86 and |8'i thereof by conductors it?, and iSd. The ampliier IE5 may be of any suitable type for amplifying an alternating voltage signal and producing an output having a definite phase relation to the `phase position of the input. For example, the ampliiier |35 may be of the same Itype as The ampliicrs are provided with Aoutput terminals `lill-l and |92 which are connected by conductors |93 and |94 to motor eld lill. Thus, the phase relationship of the current through winding il' ywith respect to that through winding |69 will be dependent upon the phase position of the output voltage oi amplier |85.

in turn is dependent upon the phase position of the voltage applied to the input terminals iS@ and |91 of the amplifier.

The voltage applied to the input terminals |95 and i9? is derived from a composite resistance network consisting of two individual networks generally indicatedby the reference numerals 29|) and 213|. The resistance network 26D consists of a transformer secondary winding 293 and two potentiometers 265 and 20%. The potentiometer 2te consists of two resistances 2% and 2de which are connected together by a conductive bar 2li). Movable over the resistances 2&3 and 239 and the conductive bar 2|@ is a slidable contact bar 2H. The bar 2H is in the form oi a bell-crank level-,the arm of which is connected to the actuating member |23. As previously explained, this actuating member E28 is in turn operatively connected to bellows iti responsive to atmospheric pressure. Thus, the position ci not only slider i i but also of slider 21| l is determined by atmospheric pressure. The connection between bellows liti and the sliders i l@ and 2| l is such that when the bellows .43 is subjected to sea level pressure, slider Hl will be at the right-hand end of its stroke and slider 2li will be at the uppermost end of its stroke. As the atmospheric pressure decreases, the sliders 2li and HS will move in a clockwise direction. At a pressure corresponding to an elevation or eight thousand feet7 slider y| i9 will leave resistance iii and will engage conductive bar |22. At the same elevation, slider 2| l will engage conductive bar 2lb. At an elevation ci thirty thousand feet, the slider iiii will move oir" or conductive bar |22 onto resistance itil and the slider 2 li will move off of conductive bar 210 onto resistance 203.

vThe potentiometer consists oi a resistance 2-l5 and a contact arm 2id movable in slidable engagement therewith. The Contact arm 2i@ is pivote'd at 2lil and yis pivotaily connected at 2|8 to `a link 2id positioned by two opposing bellows 22d and 22E. Bellows 22| is an evacuated bellows which is aiected only by atmospheric pressure. Bellows 22E) is connected by a tube 222 to the airplane cabin |52 so that the interior oi bellows '22D is subjected to cabin pressure. Due to the fact that both bellows 223 and 22E are equally affected by atmospheric pressure and since these two bellows are connected in operation, the effect of atmospheric pressure on them is cancelled out. Thus the position ci link 2| il is determined solely by cabin pressure. As cabin pressure drops, the link 2i?) is moved downwardly to move slider 2m downwardly.

The resistance 2id and the resistance assembly consisting of rcsistances '538 and 2&9 and conductive bar 2 lil are connected to secondary winding :203 to form a VVheatstone bridge. The opposite ends oi resistors 2518 and 2d@ are connected to winding 2c?. by conductors 325 and 225. The upper end of resistor 2 lli is connected to secondary winding 263 by resistor 2N and slidable contact 22M. The lower end of resistor 235 is connected to secondary winding 2633 by conductor 228. Thus, the junction of conductor '225 and resistor 221 on the one hand and the junction of conductors 226 and 228 on the other hand constitute the input terminals of the bridge. The sliders 2|| and 2|3 constitute the output terminals.

'I'he resistance network 20| includes a secondary winding 23o and a pair of potentiometers 23| and 23.2. The secondary winding 230 may constitute an additional winding of the transformer |73, or it may form part of the winding of a separate transformer supplied from the same source of power as transformer |73. The potentiometer 23| consists of a resistance member 233 and a contact arm 234 in slidable engagement therewith. The right-hand terminal of resistor 233 is connected by conductor 235 to the right-hand terminal of secondary 23e. The left-hand end of resistor 233 is connected by conductor 237 to an intermediate tap 233 of secondary 230. The slider 234 is connected through `a shaft 238 to the gear train |67 and is hence driven by motor |63. Potentiometer 23| functions as a follow-up potentiometer, as will be explained in more detail later.

The potentiometer 232 consists of a resistor 238 and a contact arm 233 movable thereover. The opposite ends of resistor 238 are connected by conductors 240 and 24| to the left-hand terminal and the tap 230, respectively, of secondary 230.

IThe slider 239 of potentiometer 232 is positioned by a proportional motor 242. Such a motor may be of any suitable type which positions a controlled object in accordance with the setting of a control potentiometer. For example, this motor` may be of the type shown in the patent to Edmondson No. 2,127,680.

The motor 242 is provided with three input terminals 243, 244, and 245. Connected to these input terminals is a potentiometer 243 consisting f of a rotatable contact arm 248 slidably engaged with resistor 247 and contact bars 270 and 27|, resistance 247 being connected in series between bars 270 and 27|. The contact arm 248 is manually operable and is provided with a knob 243 for positioning the same. The opposite ends of bars 270 and 27| are connected by conductors 250 and 25| to terminals 243 and 245. Contact arm 248 is connected by conductor 252 to terminal 244. The position which contact arm 239 will assume is dependent upon the relative voltages between terminals 243 and 244 on the one hand and conductors 244 and 245 on the other hand. This is in turn dependent upon the position of slider 248 with respect to resistor 247 and bars 270 and 27|. As shown, contact arm 248 is in its extreme lefthand position so that slider 233 is likewise in its extreme right-hand position.

The movement of arm 240 with respect to contact bars 270 and 27| and resistor 247 is limited by means of a slidable stop 272 which is suitably mounted for horizontal movement. Pivotally connected to the stop bar 242 is a link 273 which is pivotally mounted at its upper end and is designed to cooperate with an evacuated bellows 274 responsive to atmospheric pressure. A spring 275 biases link 273 into engagement with the atmospheric pressure responsive bellows 274. Upon reduction in atmospheric pressure due to the plane rising to a higher altitude, the lever 273 is rocked in a clockwise direction movingr the stop bar 272 to the left. The apparatus is so designed that up to an altitude of 8,000 feet, the stop bar 272 prevents the slider 249 from being moved in a clockwise direction beyond the end of the conductive bar 270. Between elevations of 8.000 to 30,000 feet, the stop bar 242 assumes positions in which the bar 249 can be moved a distance along the resistor 247 corresponding to the elevation of the plane above 8,000 feet. After 10 the plane reaches an elevation of 30,000 feet, the slider 249 can thus be moved over the full range of resistor 247. This, however, is the maximum amount of change that can be introduced. The slider 249 thereafter moves along the contact bar 27|.

Operation of Figure 1 The various elements are shown in the position which they assume when the plane is at an altitude oi approximately four thousand feet. Under these conditions, it is desired to maintain a cabin pressure equivalent to outside pressure, in other words, a pressure corresponding to an altitude of four thousand feet.

The air supplied to the cabin |52 under these conditions is merely what is necessary for ventilation purposes. Consequently, air should be supplied at no greater rate than it escapes-through the outlet conduit |57. Under these circumstances, the valve |04 is in a partly closed position, this position being determined by the relative positions of the contact arms of the resistance networks 200 and 20| as will be presently explained.

The various controls are shown in the position assumed when the turbine is being operated at only a portion of its capacity. In this position of the controls, the waste gate 34 is partly open as indicated by the intermediate position of contact arm 77. It is to be understood that the range of movement of contact arm 77 of rebalancing potentiometer 74 is substantially the same as the movement of waste gate 34, contact arm 77 being in its extreme right-hand position when the Waste gate is fully open, and in its extreme left-hand position when the waste gate is fully closed.

In order to understand the effect of the variation in the various conditions controlling the present apparatus, the effect of the resistance networks on the respective amplifiers will now be described. Referring 'lrst to the composite network including the individual networks 69, 70 and 7|, it will be recalled that these networks control the input voltage to input terminals B6 and i7 of amplifier 52. Each network has a denite output voltage depending upon the relative positions of its sliders. These networks are connected together in series by the following circuit: from input terminal 36 through conductor 254, contact arms 77 and 0| of network 09, conductor 255, contact ar-ms 99 and $04 of network 70, conductor 256, slider |37 and contact arm |33 of network 7|, and conductor 257 to input terminal 67 which is connected to ground as indicated at 258. The total voltage impressed between amplier input terminals G6 and 67 is thus equal to the algebraic sum of the outputs of networks 69, 70 and 7|.

For convenience in understanding the invention, the operation is treated during the single .half-cycle of operation in which the left-hand terminals of the secondary windings 73, 95 and l i7 are positive while the left-hand terminals are negative, as indicated by the positive and negative symbols appearing on the drawings.

Referring first to the network 69, it will be noted that the slider 77 assumes a potential with respect to the negative terminal which is intermediate the potential of tap 83 and Zero potential. Thus, assuming the total voltage across secondary 73 to be 42 volts, that between tap 88 on the lefthand end to be i2 volts and that between tap 93 and 36 to be 6 volts, the slider 77 assumes a potential with respect to the right-hand end which is a portion of 12 volts determined by slider 77'.

Thus. slider 11 is in a position oneefourth of the way from the right-hand end, the potential diierence between slider 11 and the right-hand end is 3 volts. The potential o slider 3i with respect to the right-hand terminal of secondary 13 is equal to the voltage existing between tap 93 and the left-hand terminal of secondary 13 plus a portion of the Voltage between the leithand end of secondary 13 and tap 93, the amount of this portion being determined by the position of slider 8| with respect to resistance Bil. As shown, slider 8| is in its extreme right-hand position in which it is at the same potential as tap 33. As slider 8l is moved to the left, the voltage between slider 8| and tap 93 is increased. The voltage between sliders 3| and 11 will at all times be dependent upon the voltage between slider 8l and the right-hand terminal minus the voltage between slider 11 and the right-hand terminal. With slider 3| in the extreme right-hand position as shown, the voltage between sliders 8l and 11 will be dependent solely on the potential between taps 93 and the right-hand terminal minus the voltage between slider 11 and the right-hand terminal. Assuming the values given above and assuming slider 11 to be in the three-quarters open position, slider 8| will be 15 volts positive with respect to slider 11.

The operation of resis-tance network 1li will be relatively obvious since, as pointed out above, this network is a simple Wheatstone bridge. If sliders SS and |04 were each in their mid positions, the voltage between them would be equal to zero, this being equivalent to a balanced condition of the bridge. Upon slider lll! assuming a position to the right of slider 951, as shown, the voltage between sliders lll and 99 is a negative voltage. If slider |04 is in its mid position and the total voltage of a secondary 95 is 24 volts, then slider |04 is exactly 12 volts negative with respect to slider 99. As a result, in the series circuit including the networks 69 and 1|) there is one 12 volt voltage and another 15 volt voltage, the two voltages being of opposite polarity. The 12 volt voltage is of such polarity as to tend to make the conductor 253 negative with respect to conductor 254, The 15 volt voltage is one which would tend to make conductor 253 positive with respect to conductor 254. As a result of these two voltages, the net voltage between conductors 2.56 and 254 is a 3 volt positive potential diierence.

The output of the network 1| is dependent upon three factors, the relative positions of the Contact arm |33, the Contact arm H9 and the slider |31. The potentiometer |35 is connected across a portion of the transformer winding ||1. This portion may be half the winding. If the total voltage of winding ||1 is 24 volts, for example, then the total voltage across resistor |35 is l2 volts. Assuming that the slider |31 is in its mid position, the potential between slider |31 and the right-hand, terminal of secondary ||1 is 6 volts. The resistors |20, |22, and |25 are connected across the entire secondary winding ||1. The voltage between slider H3 and the righthand terminal of secondary ||1 is thus a portion of the total voltage across secondary ||1, the extent of the portion being dependent upon the slider position. Under the conditions being f assumed, the slider ||.9 is at a position corresponding to four thousand feet. If, for purposes of simplicity, the resistance l2! is assumed to be equal to the resistance |20 4and the slider H9 is at approximately mid position of slider 12|, then the voltage existing between slider ||9 and the right-hand end of secondary ||1 will be 6 volts. The polarity of this voltage, moreover, will be such that the slider H9 is positive with respect to the right-hand end. The voltage between slider |19 and the right-hand terminal of secondary l1 is impressed across the potentiometer |32, so that the output of the bridge including potentiometers i8 and |35 is determined not only by the position of the sliders ||9 and |31 but also by the position of slider |33. The potentiometer |32 is employed to select the manifold pressure which is maintained. With the elements in the position shown, the slider |33 is in its mid position. Thus, the voltage between the slider |33 and the right-hand terminal of secondary |11 will be half of the voltage impressed between slider H9 and the right-hand terminal of secondary l l1. Using the specic values given above, the voltage existing between slider |32 and the right-hand terminal will be 3 volts positive. Since there is a potential diierence of 6 volts between the slider 31 and the right-hand terminal of secondary |11, slider |33 will be at a potential 3 volts lower than the potential of slider |31. Thus, the voltage output of bridge 1| as measured by the slider |32 and slider |31 is a negative 3 volts. As previously explained, the voltage output of bridge 59 as measured between sliders 8| and 11 is a positive 15 volts, and that across bridge 13 as measured between the sliders |04 and Si! is a negative l2 volts. Hence, the effective voltage across all three networks 69, 1D and 1| is Zero so that no voltage is impressed o between the amplier terminals 66 and 61.

Under the conditions just described, the pressure which is being maintained at the discharge of the compressor, in other words the induction system pressure, is at the value which has been selected by the manifold pressure selector potentiometer Let it be assumed now that the induction system pressure decreases. The effect of this is that the slider |34 is moved to the right. Such a movement to the right causes slider ll'to be more negative with respect to the slider @il so that during the half-cycle being considered the potential of terminal 61 is lower with respect to that of terminal 65, or in other words a positive voltage is applied to terminal 66. Since an alternating current is employed, this simply means that a voltage of one-phase relationship is applied to terminals 33 and 61. This signal is ampliiied by amplier 52 which supplies the winding 33 with a current displaced in phase from that flowing through winding 39. The direction of the rotation of motor 3G is such as to cause waste gate 35 to be moved towards closed position and to cause Contact arm 'l1 to be moved to the right.

The movement of waste gate 34 towards closed position tends to increase the flow of exhaust gas through the turbine 28 and hence to increase the speed of operation of compressor The movement of contact arm 11 to the right tends to increase the positive voltage difference between sliders 8| and 11. This tends in turn to eliminate the unbalance voltage of the entire composite network caused by the movement of slider ltd to the right. The movement of waste gate 34 and slider 11 will continue until the resistance network is balanced, at which time no signal voltage is applied to amplifier` 62 with the result that the winding 33 is no longer energized from amplifier 32.

If at any time there is an increase in the induction system pressure, Contact arm |04 will move to the left to raise the potential of arm |04 with respect to slider 99. This will result, during the half-cycle being considered, in the potential of terminal iii being raised with respect to that of terminal Bt. In other words, a voltage of opposite polarity is applied to the input terminals of ampliiier 62. In terms of alternating current, this is an alternating voltage of opposite phase to the alternating voltage previously considered. The effect of this is that amplifier 62 supplies a current to winding 38 which is displaced in phase by 180 electrical degrees from that previously considered. The result is that while the currents through windings 38 and 39 are again displaced 90 in phase respect to each other, the relative phase relationship is reversed. As a result, motor 35 will rotate in the opposite direction to move waste gate to closed position and to move slider 'il to the left. The eifect of the movement of waste gate Eli towards open position is to permit a greater amount of exhaust gases to flow through conduit 33 so as to decrease the amount flowing through the turbine 23. This will in turn reduce the speed of the turbine and hence the induction system pressure. The movement of slider ll to the left will tend to rebalance the network so as to overcome the signal introduced by the movement of slider |34 to the left.

If at any time, the velocity becomes excessive, the slider 8l is moved to the left. This raises the potential of slider 8| with respect to slider 'Vi so as to have the same effect upon the unbalance voltage of the entire composite resistance network as occurred when slider Hill moved to the left.

In other words, winding 38 is supplied with a current of such phase relationship with respect to that supplied to winding 3Q that the motor 4| is rotated in a direction opening the waste gate 34 and moving slider 'il to the left. The opening of waste gate 35i will reduce the speed of the turbine by permitting more of the exhaust gas to flow through conduit 33.

If the acceleration becomes excessive at any time, the acceleration controller ||l| will move slider 39 t-o the right. again be to raise the potential of terminal 61 with respect to terminal Sii during the half-cycle being considered for illustrative purposes. In other words, the effect will be the same as that produced by an increase in induction system pressure. The result is that the waste gate is again moved towards open position to decrease the amount ci exhaust gas flowing through the turbine hence to decrease the acceleration thereof.

The potentiometer $32 is employed to manually select any desired induction system pressure. Upon movement of the knob |42 in a clockwise direction, the induction system pressure that is maintained is decreased. Upon movement of the knob Mill in a counter-clockwise direction, it is increased. The movement of knob 42 in a clockwise direction, for example, causes slider |33 to be moved to the left along resistance |36. This tends to raise the potential of slider |33 with respect to slider itl and hence to raise the potential of terminal lil with respect to terminal 66. It will be recalled that this is the same effect which in every case led to a reduction in the induction system pressure. While the reduction in the induction system pressure tends to move slider |534 to the right to increase this pressure, the net effect after the system is stabilized is that the induction system pressure is at a lower value than previously existed. It will be obvious The effect of this will I4 that if slider |33 is moved to the right, the current owing through winding 38 is of the opposite phase so as to tend to increase the induction system pressure.

The purpose of the potentiometer ||8 which forms an important part of the present invention, is to limit the effect of the selector potentiometer S32. It is to be noted that when the slider |33 is moved to its extreme left-hand end of its range of movement, corresponding to the minimum induction system pressure, slider |33 is at a potential corresponding to slider I9. The position of slider i3 is in turn determined by atmospheri-c pressure. Thus, the minimum pressure position which slider |33 can assume only results in an effective position corresponding to the atmospheric pressure. As previously indicated, it is desired to always maintain the cabin pressure at least as high as the atmospheric pressure as the plane climbs up to eight-thousand feet. From eight-thousand feet to thirty-thousand feet, it is desired that the cabin pressure be the same as the atmospheric pressure at eight-thousand feet. From thirty-thousand to forty-thousand feet, it is desired that the differential between cabin pressure and outside pressure be substantially the same as that existing at thirty-thousand feet, this pressure diiferential being substantially the maximum pressure differential the cabin walls can safely withstand over prolonged periods of time. As the plane ascends, the slider ||9 is moved to the left by the atmospheric pressure responsive device 3|. The eicct of this is to increase the voltage across the resistance |34 and hence to increase the possible effect of slider |32. For example, when the slider is at the position corresponding to eight-thousand feet, it is possible for the voltage between slider |33 and the righthand terminal to be as high as 12 volts instead of the 6 volts previously considered. In other words, as the aircraft ascends from sea level to eight thousand feet, it is possible for lower and lower pressures to be selected by the manifold pressure selector potentiometer 32. At any time, however, the pressure selected by potentiometer |32 can never be lower than a value which is greater by a predetermined amount than the desired cabin pressure. During this range between sea level and eight thousand feet, this desired cabin pressure is at atmospheric pressure. It is necessary, however, to employ the compressor as previously explained to insure a flow of air through the cabin. While the aircraft is traveling between eight thousand and thirty thousand feet, the potentiometer llt has no effect upon the control potentiometer E32. This is due to the fact that the slider Mii is in engagement with the conductive bar |22 so that the potential between slider HS and the right-hand terminal of transformer Hl is at all times equal to the potential existing between the left-hand terminal 'of resistance l2l and the right-hand terminal of transformer lil. This is desirable since the cabin pressure which is to be maintained between eight thousand and thirty thousand feet corresponds to the atmospheric pressure at eight thousand feet After the plane has passed the thirty thousand foot elevation the pressure which can be selected by potentiometer 32 is again decreased due t0 the fact that the voltage which can bc impressed across resistor |34 is increased. The resistor |25) is shown as uniform along its entire length. Under such circumstances, the minimum pressure which could be maintained by the manifold pressure selector' would decrease more or less uniformlywith height. Actually, this departs slightly from the theoretically desirable condition in which the differential betweenl cabin pressure and atmospheric pressure is uniform throughout this range. Actually in practice, the departure is not material. Il it is desired to obtain exactly the desired limiting eiiect of potentiometer lit in this range, the eiiiect can be obtained by either selecting a resistor whose resistance is suitably non-uniform along its length or by employing a linkage between the bellows i3! and slider H9 which is effective to produce a non-uniform movement of slider H9 along the range of 30 to 40,000 feet.

t will be seen from the above thatl my system contemplates controlling the turbine so as to maintain a desired induction system pressure as determined by the position of a manual selector. rhe pressure which can be selected by this manual selector is, however, limited so that H it is possible to insure the maintenance of a proper pressure within the cabin.

In spite oi the fact that the pressure produced in the induction system is not lower than that necessary to maintain a desired cabin pressure, it is obvious that some means still is necessary to control the now of air through the cabin from the induction system in such a manner as to maintain the desired cabin pressure at all altitudes. My invention also contemplates means for accomplishing this. This means will now be described.

As previously explained, the motor it is employed to position a valve ltd which is located in the conduit till connecting with conduits iii and 53 leading to the airplane cabin 152. The opu eration of this motor |66 is controlled by amplifier 35 which causes winding |70 to be energized with current of a phase position depending upon the phase relation of the currents supplied to terminals |03 and iill with. respect to 'that supplied to winding 69 by the secondary E'i't'i. The voltage applied to terminals |90 and |07 is the output voltage of a composite resistance network consistlng of two individual networks 203 and 20| previously described. rlhese two networks 200 and 20| are connected in series between ampliiler terminals and i9? as follows: from terminal i953 through conductor 2&0, slider 234i, network 20|, slider 239, conductor 20|, slider 2m, network 200, slider 2li and conductors 2:32 and 2'63 back to the other terminal itil'. rThe junction of conductors 202 and 263 is preferably grounded as at 261i.

For convenience in understanding the operation of the networks, the half cycle is considered in which the lower terminal of secondary 203 is positive with respect to the upper terminal and in which the left-hand terminal of secondary 230 is positive with respect to the right-hand terminal. With the various elements in the position shown, the composite ne work is in balance. M previously noted, it is assumed that the plane is traveling at an elevation of four thousand feet. Under these conditions, the turbine pressure tends to assume atmospheric pressure which is substantially what is desired. However, in order to provide cabin ventilation7 it is necessary to maintain the airplane cabin slightly above atmospheric pressure. For this reason it is necessary that Valve |64 be slightly open. It is to be noted that the slider 2,34 is slightly to the right of its extreme leithand position which is the position it assumes when valve |64 is fully closed. The slider 239 forms part of a means for reducing the pressure when it is desired. In the arrangement shown, it is at its sea level pressure position so that it is at the extreme right-hand end of resistor 238. As a result, slider 23s is slightly positive with respect to slider 234 during the half cycle being considered. The voltage of secondary 230 and particularly that between the left-hand end of secondary 230 and tap 236 is relatively small as compared with the voltage oi secondary 203. As a result, the unbalance voltage is relatively small as compared with the unbalance voltages which may exist in connection with network 200.

Referring now to network the slider 2|| oi the atmospheric pressure responsive potentiometer 205 is at a position corresponding to an altitude of 4,600 feet. The airplane cabin pressure, however, is slightly higher than the atmospheric pressure existing at four thousand feet, this being necessary in order to obtain any cabin ventilation. Thus, the slider 2lb is close to the upper or negative end oi resistance 2|5. In spite of this, the slider 20S is at a negative potential with respect to slider 2 l0 due to the resistor 221. The effect of resistor 221 is to place the upper end of resistor EIS at a potential slightly positive with respect to the upper terminal of secondary 203. Thus, there exists across network 2&0 an output voltage which is opposite to that of network 20|. Due to the greater voltage of secondary 203, the unbalance voltage of network 200 is equal to the unbalance voltage of bridge 2M so that the two voltages cancel each other out.

Within the range of sea level to 8,000 feet, the motor |35 is not appreciably brought into operation. Let it be considered, for example, that the airplane is rising from the elevation of four thousand feet to an elevation of eight thousand feet. During this travel, the slider 2|| moves downwardly along resistor 209. At the same time, however, the cabin pressure is dropping due to the decreased atmospheric pressure outside so that slider 2| 6 is moving downwardly with respect to slider 2|5. Thus, the only unbalance voltage of network 200 is the small amount of unbalance voltage previously discussed, which unbalance voltage is necessary to cause valve |64 to be maintained in a partially open position. While the movement of slider ||9 of potentiometer H8 will tend to decrease the induction system pressure under these circumstances, this is desirable since the airplane cabin pressure is decreasing. Any tendency or the cabin pressure, however, to decrease excessively so that the desired differential between cabin pressure and atmospheric pressure does not exist results in slider 2|6 being moved downward suiliciently with respect to slider 2|| to increase the unbalance voltage of network 200. In other Words, the eiiect of this will be to make slider 2l! more negative with respect to slider 205 to cause potential of terminal |97 of ampiiier |35 to be lower with respect to terminal |96. The effect of this is to cause the motor Winding Ulti to be energized with current of such a phase position that the motor is operated to open valve li. The opening movement of valve ISG is accomplished by a movement of slider 234 to the right to increase the unbalance voltage of network 20|. The opening of valve |62 will tend to raise the cabin pressure and hence to move slider 2M upwardly to decrease the unbalance Voltage of network 200. At the same time, the unbalance voltage of network 20| is 17 increasing. As a result, the two unbalance voltages will assume equal values again, at which time the network is again balanced so that motor |56 is effectively deenergized.

While the sliders 2|| and 2|8 of potentiometers 205 and 206 tend to travel together between zero and eight thousand feet, a different condition exists between eight thousand and thirty thousand feet. During this range of altitudes, both sliders 2|| and 2|6 will move downwardly. The slider 2|| is, however, traveling over the conductive bar 2|0 so that the potential between slider 2|| and the upper terminal of secondary 203 remains unchanged. The slider 2|0 is, however, at the same time moving over the resistance 2|5 so that the voltage between slider 2|6 and the upper terminal of secondary 208 is constantly changing. As a result, the slider 2|@ becomes more positive with respect to slider 2|| increasing the unbalance voltage already existing across network 200. The effect of such an increase in the unbalance voltage has already been considered. In other words such an un balance voltage causes the motor |05 to be operated in such a direction as to open valve |84 and move slider 234 to the right. Such motion continues until the increased cabin pressure and the increase in unbalance of network 20| due to slider 234 results in the outputs of the two networks 200 and 20| being the same.

If at any time within the range of 4,000 to 30,000 feet, the plane descends, the pressure will tend to rise above the pressure desired to be maintained, namely, a pressure substantially corresponding to the pressure at an altitude of eight thousand feet. As a result, the slider 2|6 will assume such a position that the unbalance voltage tends to decrease. The effect of this is that the entire composite network is unbalanced in the opposite direction so that the potential of terminal |01 is increased with respect to that of terminal |96. This causes the amplifier |85 to supply winding with current of the opposite phase so that the motor |60 drives valve |64 and slider 234 in directions opposite to that previously considered. In other words, valve |64 is moved towards closed position to decrease the flow of air to the cabin and slider 234 is moved to the left to decrease the unbalanced voltage of network This continues until the increase in unbalance of network 200 resulting from the decreased cabin pressure acting on slider 2|0 and the decrease in unbalance voltage of network 20| due to movement of slider 234 causes the unbalance voltages to be equal again. As previously noted, the eifect of potentiometer 23| is relatively small as compared with the effects of potentiometers 205 and 206. Hence, even though slider 2lb returns to a position corresponding substantially to the desired pressure, the small change in unbalance voltage introduced by the deviation of this slider from this position is sufficient to offset the change in unbalance of network 20| necessary to produce the desired change in position of valve |54. Stated in other words, a very small change in the position of slider 2|6 is necessary to maintain a desired change in the position of valve |84.

Upon the aircraft ascending above 30,000 feet, the potentials of both sliders 2|| and 2|0 with respect to the negative terminal of secondary 203 are changed, just as in the range of sea level to 8,000 feet. Thus, as in the range of 4,000 to 8,000 feet, there is a tendency for the pressure lli) to decrease with atmospheric pressure. It is to be understood, however, that at the elevation of 30,000 feet the pressure maintained in the cabin is the pressure existing at 8,000 feet so that while the pressure in the cabin continually decreases, the differential between cabin pressure and outside pressure remains constant.

It 'will be seen from the above description that with the control system shown, the pressure is maintained at substantially atmospheric pressure between sea level and 8,000 feet, that a pressure corresponding to atmospheric pressure at 8,000 feet when the aircraft is between elevation of 8,000 to 30,000 feet, and at a pressure when the aircraft is above 30,000 feet such that there is a differential between cabin pressure and atmospheric pressure equal to the differential which existed at 30,000 feet.

In the preceding description, the function of potentiometer '232 has not been described. As previously indicated, this potentiometer is provided for the purpose of decreasing the cabin pressure below that automatically selected. Potentiometer arm 239 is shown in the drawing in the position in which it is normally set. When it is desired to lower the pressure, the knob 249 of the manually operated potentiometer 246 is rotated in a clockwise direction. Such rotation of the knob 249 initially has no effect upon the proportioning motor 242 since slider 248 is simply moved along the conductive bar 210. As previously pointed out, it is impossible by reason of the stop bar 212 for the slider 248 to be moved beyond the conductive bar 2l0 until an altitude of over 8,000 feet has been reached. Let it be assumed that the craft is traveling at an altitude of 12,000 feet. Under normal conditions, the cabin pressure control system is operating to maintain within the cabin a pressure corresponding to an altitude of 8,000 feet. In other words, there is a pressure differential between the interior of the cabin and the atmosphere corresponding to an altitude differential of 4,000 feet. If the operator now desires to reduce this pressure, the knob 249 is rotated so that the slider 248 goes over conductive bar 210 onto resistance 241 to an extent corresponding to the desired reduction. This causes the voltage between conductors 250 and 252 to be increased and that between conductors 25| and 252 to be decreased. As a result, the timing motor is caused to move the slider 239 with respect to resistor 238. This movement does not occur, however, as rapidly as the slider 248 is moved due to the fact that the motor 242 is designed to move slider 239 at a slow predetermined rate. In one typical system the motor 242 is designed so as to move slider 239 at a rate corresponding to an altitude change of 1200 feet per minute. Such an altitude change is the maximum altitude which can be permitted without undue discomfort to certain of the occupants of the aircraft. Thus, upon slider 248 being moved to the right with respect to resistance 247, the slider 239 is slowly moved to the left. Such a movement of slider 239 results in the slider '239 becoming more positive with respect to the slider 234. It will be recalled that this is the effect which results in the motor rotating the valve |54 to closed position. The resulting reduction in pressure tends to move slider 2|0 downwardly with respect to resistor 2|5 to cause the unbalance voltage of network 200 t0 be increased, This tends to cause the composite network to be rebalanced. At the same time as the motor is moving valve |34 to closed position, the slider 234v of the rebalancing potentiometer 23| is moving to the left so as to decrease the unbalancing of network 20|. The combined effect of the rebalancing potentiometer 23| and that of cabin pressure responsive potentiometer 233 results in the entire network being balanced with the cabin pressure at a lower value than it previously assumed. The portion of the secondary 230 across which potentiometer 232 is connected is so selected in value with respect to the portion of secondary 230 across which potentiometer 23| is connected that the movement of slider 239 across the full length of resistor 238 will produce a maximum cabin pressure change equivalent to the difference between atmospheric pressures at 8,000 to 30,000 feet. Thus, when the plane is operating at 30,000 feet and the stop rod '212 is withdrawn suiciently far to permit slider 248 to travel the full length of resistance 201 so as to permit slider 239 to travel the full length of resistance 238, the reduction in pressure in the cabin which is aiected by such travel is a reduction in pressure from the pressure existing at 8,000 feet to that existing at 30,000 feet. With this arrangement, it is accordingly possible at that time to reduce the cabin pressure from the pressure then being maintained to any desired lower pressure so long as the desired pressure is not below atmospheric pressure. Furtherrnorey such a change in the cabin pressure will be eiected at a timed rate which would not produce severe discomfort to the occupants of the cabin.

Species of Figure 2 The species of Figure 2 differs from that of Figure 1 in the control means for the cabin pressure and the manner in which this control means is employed to inuence the induction system pressure. In the arrangement of Figure 2, the flow of air to the cabin is controlled to provide a relatively constant flow and the flow of the air out of the cabin is controlled to provide the desired pressure within the cabin. Furthermore, a somewhat diierent means is provided for controlling the manual reduction in cabin pressure and for limiting the pressure which can be selected.

In view of the fact that most of the apparatus for controlling the waste gate associated with the turbine is identical to that employed in Figure 1, the same reference characters have been employed in referring to elements which are identical to those in Figure l. Furthermore, these elements are not described again in connection with Figure 2.

The reference numeral 3|8 is employed to designate a potentiometer similar in function to potentiometer ||8 of the species of Figure 1. This potentiometer comprises a slider 3|9 movable in slidable engagement with a resistance element 320 which is connected by conductors 323 and 324 to the opposite ends of secondary ||1. The slider 3|9 is connected by a threaded shaft 326 to a gear train 321 which is driven by a motor 328 in a manner which will be subsequently explained. The manual selectior potentiometer |32 in the present species is connected between slider 3|9 and the right hand terminal of secondary ||1, in the same manner as in the species of Figure 1. by conductors 330 and 33|.

In the present species, a portion of the cabin is shown in section, the cabin wall being indicated by the reference numeral 335. The conduit |5| leads through this cabin wall 335. A motor operated valve 338 controls the iiow of air through conduit |50. It is to be understood that a similar valve (not shown) is provided in connection with conduit l53 leading from the supercharger of engine #3. The valve 338 is operatively connected to a motor and gear train 339 which is energized from the secondary winding 340 associated either with the main transformer d4 or some other transformer connected to any suitable source of power. The motor and gear train 339 are provided with three control terminals 3fil, 332 and 343. The connections of these terminals are controlled by a differential pressure responsive switch generally indicated by the reference numeral 358. This switch comprises a switch blade 35| movable between contacts 352 and 353. The switch blade 35| is connected to a link 35A which extends between two opposed bellows 355 and 355i. Bellows 356 is connected to a conduit 351 which extends through the cabin wall 335 so as to subject the interior of bellows 356 to the cabin pressure. The bellows 355 is connected by a conduit 358 to a portion of the discharge conduit |50 spaced a substantial distance from the cabin so that the diierence in pressures in bellows 355 and 35S corresponds to the pressure drop through a section of conduit |50. The differential pressure system switch 350 operates to control the energization of motor 33S so as to control valve 338 in such a manner as to maintain a constant ow of air through conduit |50, as will be eX- plained in more detail in connection with the operation.

A conduit 330 extends from the interior of the cabin to the atmosphere to provide for escape of the air from the cabin. A valve 35| is located within thisv conduit and is operatively connected to a gear train 302. The gear train in turn is driven by a motor 353. The motor is shown as being of the induction type having a rotor 364 and a pair oi eld windings 355 and 365 which are connected together and are connected to ground by a conductor 381. The motor winding 305 is connected to the output terminals 353 and 380 of an amplifier 310. rIhis amplifier is of the same type as amplifiers 52 and |85 in the species or Figure 1. In other words, the amplifier' is of a type which will produce an output voltage having a phase relation depending upon the phase relation of the input voltage. rFne ampliiier has, besides the output terminals 358 and 359, power supply terminals 3'52 and 313 and input terminals 314 and 315.

The amplifier and motor are energized by means of a transformer 300. This transformer comprises a primary winding 38| and a pair of secondary windings 302 and 383. The primary winding 38| is connected to any suitable source of alternating power (not shown). The secondary winding 382 is connected by means oi conductors 33t and 385 with the input terminals 312 and 313. IIhe secondary winding 383 is connected by means of conductors 385, 351 and 381 to the opposite terminals of eld, winding 306. Thus, the motor field winding 336 is constantly energized by current of a iixed phase. The other iield winding 355 is energized by the ampliiier with a current whose phase position is dependent upon the phase position of the Voltage applied to the input terminals 314 and 315 of amplier 310.

The voltage applied to the input terminals 314 and 315 is the output voltage of a composite re- Sistance network consisting of individual networks 390 and 39|. The network 390 is in the form of a bridge consisting of a secondary winding 392 and two potentiometers 393 and 394. Secondary winding 392 may constitute an additional winding of transformer 389 or the winding of a further transformer energized from the same source of power as primary 38|. The potentiometer 393 comprises a resistance 395 and a slider 39B. The slider 39B is connected by means of a shaft 391 to the -gear train 362 so as to be moved Whenever the motor 363 is operated to move the valve 36|. The potentiometer 393 operates as a rebalancing potentiometer in a manner which will be fully described later. The potentiometer 394 constitutes the main controlling potentiometer and consists of a resistor 3% and a slider 399. The slider 339 is positioned by means of a bellows 409 which is evacuated and is located within the cabin so as to be responsive to cabin pressure. The bellows 599 operates against a spring 49| which is adjustable by means of a screw 492 extending through a suitable support 403. Upon any increase in cabin pressure, the slider 399 is moved by the spring 49| to the right along the resistance 398. Upon a decrease in cabin pressure, the bellows 499 expands and moves slider 399 to the left against the action of spring 49|.

The network 39| consists of a secondary 4|@ and a further potentiometer 4H. Again, the secondary 4|@ may constitute an additional secondary of transformer 389 or a secondary of another transformer providing that it is energized by the same source of alternating current as is primary winding 38|.

The potentiometer 4H consists of a resistor 4|2 and a slider M3. The slider 4&3 is actuated by timing mechanism of the type more fully disclosed in the application of Albert E. Baak, Serial No 541,583, filed of even date herewith. This timing mechanism is shown only schematically in the present application. The slider M3 is secured to a block 4H! which has a threaded opening through which extends the threaded rotatable shaft 326 connected to gear train 321. The block H4 carries a pair of resilient contact blades 4|6 and 4|1. Cooperating with blades M6 and 4|1 are a pair of blades 4|8 and 419, respectively. These blades are secured to the opposite sides of a pointer 429 which is rotated by means of a knob 42 I. The pointer 529 cooperates with a scale 52 The motor 328 referred to above is a direct current motor having a wound armature @25 with which are associated the brushes 523 and 6121. Also cooperating with the armature @25 are a pair of iield windings 425 and 43d. The motor rotates in either direction, depending upon which winding is energized. Connected in series with the rotor 425 and controlling the ow of current therethrough is a rheostat consisting of a resistor 32 and a contact arm 433 movable over the resistor 432. rThis rheostat is employed to vary the current ilowing through the armature 425 and hence to vary the speed of operation of motor 328.

A battery 435 is employed to energize the motor. Any other conventional source of direct current may be employed. A manually operable switch 435 controls the connection of the battery to the motor.

A differential pressure switch is generally indicated by the reference numeral This switch consists of a switch blade 45| movable between two opposed contacts 442 and 443. Switch blade 4M' is secured to a pivoted arm 445 which is urged into engagement by a spring 4M with bellows 246. The bellows 44B is provided with a spring 441 therein. The interior of the bellows, being located in the cabin, is -subjected to cabin pressure. The interior of the bellows is subjected to atmospheric pressure in that it is located adjacent to an opening 448 extending through the Wall 335 of the cabin.

Operation of species of Figure 2 As previously indicated, the valve 338 in the line |59 is so regulated as to maintain a constant flow of air to the cabin as long as the pressure in the induction system is adequate to maintain the desired cabin pressure. As will be explained hereinafter, means are provided to insure that the induction pressure will always be maintained at least at such a value. The valve 338 is positioned by motor 339 which is controlled by the dierential pressure switch 359. With the elements in the position shown, the switch blade 35| occupies a position intermediate contacts 352 and 353. Upon a decrease in the pressure drop through conduit |59, the switch arm 35| is moved into engagement with contact 352. This causes terminals 342 and 343 to be connected through conductors 45), switch blade 35|, contact 352 and conductor 45|. Whenever terminals 342 and 3fm are thus connected, the motor is operated in such a direction as to cause valve 338 to move towards open position. This tends to reduce the impedance to air flow so that the ow of air is increased. Whenever the pressure drop through conduit |59 increases indicating that an increase in the flow of air through conduit E59 is taking place, the Switch 35| is moved into engagement with contact 35,3 establishing a circuit between terminals 342 and 34| as follows: from terminal 342 through conductor A59, switch blade 35|, contact 353 and conductor 452 back to the other terminal 34|. Whenever terminals 352 and 311| are thus connected, the motor tends to operate in such a direction as to move valve 355 towards closed position to thus reduce the flow of air through the conduit 59. Thus, the vaive is moved in one direction or another upon any change in the pressure differential between the junction of conduits |59 and 358 and the cabin so as to maintain a relatively constant flow of air through conduit |59 regardless of such change in cabin pressure.

The apparatus for controlling the pressure within the cabin will now be described. As previously indicated, the voltage applied between the amplifier input terminals 314 and 315 is the unbalance voltage of the composite network consisting of networks 399 and 39|. These two networks are connected between terminals 314 and 315 in series as follows: from terminal 315 through conductor Q55 to the center tap 459 of secondary lll, through the network 39l, contact arm M3, through conductor 451, slider 393, the network 399, slider 396, and conductor 558 to the other input terminal 315. The various elements of the cabin pressure control system are shown in the position assumed when the airplane cabin is at the desired pressure intermediate between sea level pressure and atmospheric pressure. With the system of the present invention, this pressure is not automatically changed but is maintained constant until changed by means of a manual actuator constituting a part of the timing mechanism previously referred to.

With the elements in the condition shown, the composite network is balanced due to thev fact 23 that the network 390 is balanced and the slider 413 is in a position substantially midway between the ends of resistor 4l 2. it be assumed now that for some reason or other the pressure within the cabin drops. This will cause a movement of slider 399 to the left. If the conditions are examined during the hali-cy^le in which the left-hand terminal of secondary 392 is positive with respect to the right-hand terminal, this movement of slider 399 tends to cause it to become positive with respect to slider 399. This in turn will cause the potential of terminal 314 to be raised with respect to terminal 315. It is of course to be understood that actually an alternating voltage is applied between terminals 315 and 314 due to the fact that secondary 392 supplies an alternating voltage to the network 39e. This alternating voltage is of such phase relationship that the motor 363 is operated in a direction such as to cause movement of valve 36E towards closed position so as to decrease the escape of air from the cabin. This movement or" valve Bei will be accompanied by the movement of slider 395 to the left so as to tend to rebalance network 3953. At the same time, the eiTect of the change of the pressure in the cabin will iniiuence bellows 49d to cause slider 399 to return in the direction of the right-hand terminal of resistance 399. The combined eiects of rebalancing potentiometer and control po-l tentiometer 394 will result in network 39d being rebalanced so that no voltage is applied between terminals 315 and 3'54. The motor will then be deenergized.

If, on the other hand, there is a rise in cabin pressure, the slider 399 will be moved to the right so as to cause an unbalance of the network 390 in the opposite direction. The alternating voltage applied between terminals 314 and 315 will accordingly be opposite in phase from that applied with the conditions previously considered. The motor SES will hence be rotated in the opposite direction to move valve 39! to open position so as to permit a greater escape of air from the aircraft through conduit 36e. At the same time, slider 396 will -be moved to the right tending to rebalance network 399. Again, due to the change in pressure in the cabin which will affect the slider 394 due to the action of rebalancing potentiometer 393, the network 399 will be rebalanced when a movement of valve 3S! necessary to effect the desired change in cabin pressure has taken place.

It will thus be seen that normally the cabin pressure is maintained at a constant value by operation of the discharge valve SBI. However, as pointed out in connection with the species of Figure 1, it is desirable that the cabin pressure be changed as the plane operates in different altitudes. Whenever the pilot desires to change the cabin pressure the knob 42! is rotated in one direction or the other depending upon the desired direction of cabin change. When the knob 42! is rotated in a clockwise direction, the cabin pressure is decreased and when it is rotated in a counterclockwise direction, the cabin pressure is increased. Let it be assumed rst that it is desired to increase the cabin pressure. In such event, the knob 42! is rotated to bring the switch blade 4I8 into electrical engagement with the switch blade M6. The switch blades 416 and 4I!) are resiliently secured to block 454 at their upper ends so as to be lfreely yieldable. The pointer 420 is rotatably secured on a shaft through a slip friction connection so that upon pointer 429V being moved in either direction it will remainv in the position to which it is moved until it is again manually moved. In other words. it isv capable of remaining in the position to which it is moved against the tension of either switch blade 4l@ or 4i? as the case may be. Thus, switch blade iid is maintained in electrical connection with switch blade 4!6. The engagement of these two switch blades causes a circuit to be established to motor field winding 429 as follows: from the positive terminal of battery 435 through conductor 459, switch blade 44|, Contact 443, conductor 459, contact blades 4|8 and 4|6, conductor 4t2, field winding 429, conductors 463 and 494, switch 435 and conductor 465 to the negative terminal of battery 435.

The energization of motor field winding 429 causes motor 329 to move the block 4! 4 in such a direction as to move slider 45S and contact arms MG and M1 to the right. This movement will take place at a timed rate dependent upon the setting of the rheostat consisting of resistor 432 and contact arm 433. After there has taken place a movement of block H4 corresponding in magnitude to the original movement of pointer 429, switch blade Mil will separate from switch blade 418 so that the circuit traced to eld winding 429 will be interrupted at which time further movement of block 4M will stop. As a result of the movement of block M4, the slider M3 will have been moved to the right with respect to resistor 4I2 a distance corresponding in magnitude to the movement of pointer 429. The secondaries 4H) and 392 are so associated with the source of supply that the left-hand terminals are always positive with respect to the righthand terminals at the same time. Thus, during the half-cycle previously considered, the movement of slider 453 to the right will cause the center tap 455 to become positive with respect to slider 4|3. The effect of this is to produce an unbalance voltage of the same character as eX- isted when the pressure within the cabin decreased. It Will `be recalled that the phase of the unbalance voltage in such case resulted in the amplifier energizing the field winding 395 in a manner to cause the motor 3&3 to move the valve 36| towards closed position, thereby permitting less escape of pressure from the airplane cabin. Thus, the eiect of moving pointer 429 to the right is to cause valve 36E to be moved to closed position to increase the pressure in the cabin. This decrease in pressure in they cabin will cause a movement of slider 399 to the left. At the same time, an increase in cabin pressure will cause slider 399 of the pressure responsive potentiometer 394 to be moved to the right. These two actions result in an unbalance voltage which when a certain position of valve 36! is reached will be equal to the unbalance voltage caused by movement of slider M3. Thus, valve 35| will be moved to a position corresponding to the new position of slider M3, at which position the new higher pressure is maintained. The increase in pressure, moreover, has been accomplished at a gradual rate dependent upon the rate of movement of slider 4! 3 by motor 32S. This rate of change of pressure is determined by the setting of rheostat slider 433 with respect to resistor 432.

If on the other hand, it is desired to decrease the cabin pressure, the pointer 429 is rotated to the left so as to bring contact blade 4i!) into electrical engagement with contact blade 4H. Again, the blade 4H will deect by an amount necessaryto permit the desired movement of pointer 420. An energizing circuit to field winding 430 will now be established, this circuit being as follows: from the positive terminal of battery 435 through conductors 410 and 41|, contact blades M9 and 4|1, conductor 413, field winding 430, conductors 414 and 464, switch 436, and conductor 465 to the negative terminal of battery 435. With motor eld winding 433 energized, the motor 328 will rotate in a direction opposite to that previously considered so that the block 4|4 will move to the left. This will cause movement of slider 4|3, switch blades 4|6 and 4|1 to the left. Such movement will continue at a timed rate until contact blade 4|1 has separated from Contact blade 4| 9. The slider 4|3 will now be in a new position dependent upon the amount of movement of pointer 426, this position being to the left of that previously occupied. The effect of the movement of slider 4|3 to the left is to create an unbalanced voltage opposite to that previously considered so as to cause the motor 363 to rotate the valve 36| towards open position. The rotation of valve 36| towards open position causes a decrease in the cabin pressure which in turn will cause a movement of slider 369 to the left. At the same time, the slider 396 of the rebalanced potentiometer 333 is moved to the right. The effect of this is that the network 390 is unbalanced and when the valve 36| has reached a position corresponding to the movement of slider 4|3 to the left, the unbalance voltage of network 3923 will be equal and opposite to the unbalanced voltage of network 39|. When this condition exists, the cabin pressure will be at a lower value corresponding to the new setting of slider 4 3.

It will be seen from the above that provision is made for manually adjusting the cabin pressure at a desired rate either upwardly or downwardly. Provision is at the same time made for insuring that the differential between cabin pressure and atmospheric pressure can never become excessive. Otherwise, it might be possible to cause the existence of a cabin pressure of such a high value that the-cabin walls would not be able to withstand the diierential between cabin and atmospheric pressure. The differential pressure switch 446 is provided for this purpose. Normally, the switch blade 44| of the differential pressure switch is in the position indicated in which it is in engagement with contact 443.

Let it be assumed, however, that the differential pressure becomes excessively high, either through the operator selecting a high pressure by the mechanism just described or by reason of the plane ascending to a relatively high altitude while a constant pressure is being maintained therein. Under these conditions, the switch blade 4| will move from engagement with contact 443 into engagement with contact 442. This will do two things. In the first place, a circuit traced through winding 429 will be interrupted so that it will be impossible to maint-ain this circuit by the engagement of contacts 4|6 and 4| 8. Thus, if the undue differential in pressure has been. caused by the operation or the adjusting mechanism, the circuit to motor 32B causing the undesirable adjustment is immediately interrupted. At the same time, a circuit is established to the other motor eld winding 436 independent of the manual operator as follows: from the positive terminal of battery 435 through conductor 463, switch blade 44|, contact 442, conductor 416, eld winding 431), conductors 414 and 464,

the induction system negative terminal of battery 435. The energize,- tion of motor field winding 430 causes the motor to be energized in such a manner as to cause block 4|4 to be moved to the left which also results in the movement of slider 4|3 to the left. As previously pointed out, however, such a movement of slider 4|3 causes an unbalance in composite network in such a manner as to cause a reduction in the air pressure within the cabin. Such movement of the block 4|!! will continue until the pressure within the cabin has dropped sufficiently to move switch blade 44| out of engagement with contact 442. Thus the differential pressure switch 443 operates automatically to insure against an excessive differential between cabin and atmospheric pressure.

The 'slider 3&9 of potentiometer 3|8 is moved simultaneously with the slider 4|3 of potentiometer lill whenever the manual adjusting mechanism is operated to change the position of the latter slider. Hence, the positions of sliders 3|9 and 4|3 always correspond. As will be recalled from the description of the species of Figure 1, the analogous potentiometer ||8 was employed to limit the minimum pressure which could be set by the selector potentiometer |32. The potentiometer M8 is provided lfor the same purpose. In the species of Figure l, the cabin pressure was adjusted in accordance with atmospheric pressure and the potentiometer ||8 was likewise adjusted. In the present case, the cabin pressure is adjusted in accordance with the manually operable timing mechanism described above. The potentiometer 3|8 is accordingly so adjusted. Any movement of slider 3|!! to the left will decrease the induction system pressure as long as slider |33 is to the left of its extreme right-hand position. This is desirable since a reduction in cabin pressure makes it usually possible to lower pressure. If for any reason the induction pressure is too low for engine operation, the manual selector |32 can be adjusted to maintain a higher induction system pressure. Conversely, when slider 3|!! is moved to the right, the induction system pressure that is maintained is increased. This again is also desirable since the slider SIS is moved in this direction at the same time that the slider 4|3 is moved in a similar direction to increase the cabin pressure. Again, if a higher cabin pressure is necessary, it is usually necessary to raise the induction system pressure. Thus, means are provided to limit the minimum induction system pressure which can be selected in accordance with the cabin pressure which is to be maintained.

Because of the tendency for the cabin pressure and induction system pressure to be adjusted simultaneously, the change in cabin pressure normally has no appreciable effect upon the operation of valve 338 by differential pressure switch in other words, while the cabin pressure is lowered the induction system pressure is likewise lowered so that the differential switch 350 is relatively unaifected. The di'erential pressure switch is operative to adjust the position of valve 338 primarily when there is some abnormal fluctuations in either the induction system pressure or the cabin pressure, the adjustment being made for the purposes of maintaining a uniform flow rate rather than for affecting the cabin pressure.

Species of Figure 3 The species of Figure 3 is concerned with means manual switch 436, and conductor 465 to the It for compensating the operation of the induction system pressure regulating means for the various engines as the throttles are adjusted. As has been previously noted, the cabin pressure is maintained by the superchargers associated with the No. 2 and No. 3 engines. Furthermore, as has been pointed out, some means indicative of the cabin `pressure is employed to insure that the manual selector of the induction system pressure cannot be operative to select a pressure below that required to maintain the cabin pressure at the desired value. It will be obvious that under many conditions of operation, the induction system pressure and consequently the manifold pressure that results is higher than that desirable from a standpoint of engine operation. Consequently, it becomes necessary for the operator to retard the throttle of the associated engine. This ,tends to cause a reduction in the amount of exhaust gases available for operation of the turbine so that for a given Waste gate setting, the induction system pressure maintained will be lowered. As will be noted from the description of the induction system pressure control arrangement of Figure l, the system tends to maintain a given waste gate position for a given induction system pressure. Obviously if the amount of exhaust gases is materially decreased, the pressure that is maintained in the induction system will correspondingly decrease due to the lack of motive power. Hence, for a given setting of the main selector a lower induction system pressure will tend to be maintained. On the other hand, in those engines whose throttles are not retardedy an excessive induction system pressure will result. In the arrangement of Figure 3, means are provided for lowering the pressure settings of the induction system regulating means for those engines whose thrcttles are retarded and `for raising the settings of the regulating systems for the other engines. Specically, the compensating means isoperated in conjunction with the throttles of they No. 2 and No. 3 engines, the engines whose superchargers are employed to pressurize the cabin. Furthermore, in the arrangement of Figure 3, the means for preventing the induction system pressure from being lower than a predetermined value is responsive directly to cabin pressure instead of to atmospheric pressure as in the species of Figure 1 or to a position of a manual adjustor as in the species of Figure 2.

The induction system control networks for the various engines are identical with that shown and described in connection with Figure l, with the exception of the addition of the compensating potentiometers. The elements of the induction system control network in engine No. 2 which are identical to those of Figure l have been given identical reference characters, The elements for the induction system control networks of engines No. l, 3 and 4 have been given the same numerals with the suffix letters a, b, c, respectively. With this understanding, it is believed unnecessary to specically describe these previously explained portions of the induction system networks.

Referring rst to that portion of the system which is common to the control systems of the various superchargers, there is connected across the secondary Il a potentiometer 5|8 which corresponds in function to the potentiometer 8 of the species of Figure l and M8 o f the species of Figure 2. The potentiometer 5|8 comprises a slider 5|9 anda resistor 52B.

terior of the bellows 524 is connected by a tube 525 to the interior of the cabin (not shown). Bellows is thus at all times subjected to cabin pressure. The bellows 523 is an evacuated bellows which affected only by atmospheric pressure. Since both oi the bellows 523 and 524 are equally aiected by atmospheric pressure, it will be lobvious that the position of link '522 is not affected by atmospheric pressure so that the position of link and hence of slider 5|9 is determined solely by the cabin pressure. One terminal of a. resistor 231i of the pressure selecting potentiometer |33 is connected by a conductor 526 to the slider 539 of potentiometer 5|8, The other terminal or resistor |34 is connected by a conductor iil to the lower terminal of resistor 29 of the potentiometer 5|8. The slider |33 is connected by a conductor 529 to ground at 528. Since input terminals 67, Sia, 61h and Sie of the amplin- $22, 62a, till), and 62e are also grounded, the slider E33 is hence connected to each of these terminals.

It will -be obvious Ithat the potentiometer 5|8 has the same effect as potentiometer ||8 of Figure l Iand 3i 8 of Figure 2. The maximum Iamount of voltage change that can be introduced by the pressure selector :potentiometer |32 is dependent upon the I'position of slider 5|9. It will be recalled :that the movement `of slider |33 to the left causes `a decrease in the induction system pressure by moving the waste gate towards open position. Obviously, the further slider 5| 9 can move towards the upper end of slider 520, the greater can be the effect of potentiometer |32 when slider |33 is moved :to the left. Hence, the further slider 5|9 is moved upwardly, the lower will 4be the pressure which can be selected by potentiometer |32. Upon a decrease in cabin pressure, the slider 5|9 is moved upwardly so as to permit the induction system pressure to be lowered further by the ypressure selector potentiometer |32. Similarly, Whenever the cabin pressure is raised, the minimum induction system pressure that can be selected is also raised. Not only is the value of the minimum pressure raised but unless slider |33 is at its extreme righthand end `the movement of slider 5| 9 tends to effect an immediate change in the value of the induction system pressure.

In Figure 3, no particular means is shown for controlling cabin pressure. It is to be understood that either the arrangements of Figures 1 or of Figure 2 could be employed. Other means for controlling the cabin pressure could also be employed if desired.

As h-as been previously explained, the secondary ||1 and lthe pressure selecting ypotentiometer 32 constitute a part of all of the control systems vfor the tur-bo superchargers. Except for the secondary the pressure selector potentiometer |32 and the limiting potentiometer 5|8, the control system for each engine is separate and distinct. Referring now to the control sys- 'tem for the No. 2 engine, the elements of this `Systcrrl are identical to :that shown in Figure 1 with the exception of the addition of a further network 533 which is interposed between the calibrating potentiometer |35 and the control potentiometer 94. This network 530 comprises a secondary 53| and a plurality of potentiometers 532 and 533. The potentiometer 532 is a, manually operable adjustable potentiometer and comprises a resistance 5.3.4 and Aa slider 5.3-6. The resistance 53.4 is connected by yconductors 542 and 543 to the opposite terminals `of secondary enclaves 53|. The potentiometer 533 consists cfa resistor 531 terminating .at *its right-hand end in a conductive bar 53B. Also included in the potentiometer 533 is a slider 539 pivotally mounted at 545 and movable in slidable eng-agement with the conductive bar 538 and the resistor 531. A link 54| extends between the slider 539 and the throt-tle lever 23. The throttle |16 is illustrated in its `wide open position. Upon the throttle lever 23 being moved to the left to move throttle I6 towards closed position, the slider 539 is moved to the left with respect to resistor 531. The effect of potentiometer 533 is determined by the setting of slider 536, one end of resistor 531 being connected by conductor 544 to the :slider 536 and contact bar 533 being connected by conductor 545 to the right-hand end of resistor 534.

Referring now to the control system for engine No. 1, the portion Ioi the system' which is concerned solely `with the control of that engine differs from the corresponding portion of the system shown in Figure 1 only in the position of a network 55) between the Calibrating .potentiometer |4| and the control potentiometer 94a. This potentiometer 55e likewise comprises a secondary 55| and two .potentiometer-s 552 Iand 553. The :potentiometer 552 is a manually adjustable potentiometer consisting of a resistor 554 and a slider 556. The resistor 55d is connected by conductor 56d and 555 to the opposite terminals of secondary 55l. The left-hand terminal of secondary 55! is further lconnected by a conductor 556 to the slider of calibrating potentiometer !4|. The potentiometer 553 consists of a resistor551, a conductive bar `and -a slider 559 movable with respect to the bar and the resistor. One end of resistor 551 is connected by aconductor 553 to the slider 555. The contactbar 558 is connected by a conductor 561 to the left-hand terminal of resistor 554. Slider 5|9 is also connected =to the throttle lever 23 for engine No. 2. A connection is effected by means vof two links 55|] and 55|, and a lever 552 pivoted :at its mid point. The link 56D connects one end of lever 552 to the .slider 559 and the link 56| the Iother end of slider 552 to lever 23. It will be obvious that upon movement of lever 23 to Ithe left to clos-e Ithrottle it, lever 55B is moved to the right.

Consideringr now the control system for engine No. 3, this is .similar to that for engine No. 2, the only difference being that instead of the resistance network 55S, there is connected into the system network Each of the elements of this network are identical to the elements of the network and have been given the same numerals with the suiiix letter b to indicate their use in connection with engine No. 3. It is believed unnecessary to describe these elements in detail. It is to be noted, however, that the slider 5395 is connected to lever 25h and is actuated in the same direction and in the same manner as slider 539 upon throttle lever 23o being moved towards throttle closed position.

Considering now the controls for engine No. 4, these controls include a network 550e which corresponds in arrangement and function tothe `network 555 in connection with the controls for engine No. l. Again, the elements of this network are identical to the elements of network 55! and so the same reference numerals have been employed in referring to these elements as were employed in referring to the elements of network 555, the sunix letter c being added tothe numerals in this case to denote their Ause rinconnection with the controls for engine No. 4. As

with network 55B, the slider553c is movedto the Operation of species of Figure 3 The, various controls are shown in the position assumed when the throttles for both the No. 2 andNo. 3 engines are entirely open and when the cabin pressure is at some intermediate value. Under these conditions, the amount of voltage that can be introduced by the pressure selecting potentiometer |52 is limited by the position 0f slider 559, as previously explained. If a lower` cabin pressure is selected through either an automatic or through some manually operated selecting mechanism such as shown in Figure 2, the slider 515 will move upwardly to permit the selection of a lower induction system pressure. If the cabin pressure is raised, on the other hand, the slider 5|@ will move downwardly to prevent the selection of as low an induction system pressure. The pressure selector potentiometer, as also previously explained, aiects the control systems for all four engines so that any change in the pressure selected either by action of pressure selecting potentiometer S32 or the limiting potentiometer 553 causes an equal change in the pressures maintained in all of the engines.

It will be obvious that since the minimum pressure which can be maintained is dependent upon the cabin pressure, it may well often happen that the manifold pressure is higher than that desired. As a consequence, it is necessary for the pilot to retard the throttle in order to increase the pressure drop between the induction system and the inlet to the direct driven compressor, so as to decrease the manifold pressure to a value desirable for engine operation. Such a reduction in the throttle position decreases the amount of exhaust gases, however, and tends to cause a lower induction system pressure for a given setting of the waste gate. t will be recalled from a description of the control arrangement for the induction system in connection with Figure 1 that if the other po-tentiometers are not adjusted the control system tends to maintain a given waste gate position for a given position of the main control potentiometer 94, which in turn assumes a denite position corresponding to induction system pressure. In other words, if for any reason the induction system pressure does drop, the waste gate is moved towards closed position but is maintained in a more nearly closed position only so long as theinduction system pressure is lower than that desired. Thus, while the departure from the desired .induction system pressure is small, there is nevertheless a delinite departure whenever the conditions are such as to require waste gate position other than the average Waste gate position selected as the operating point. This necessary departure of the controlling condition from the desired value upon a change in the load is a characteristic of any proportioning system and is commonly called the droop It will be obvious thatanything which demands that the waste gate go to an even more closed position in order to maintain a given induction system pressure will increase this droop The closing of the throttle with the resulting reduction in the total amount of exhaust gases will obviously increase .this effect. While a certain amount of droop is desirable in connection with airplane manifold pressure control systems since it is desirable for the npulpose .of maximum power to. maintain lower manifold pressures asv higher altitudes are reached, the amount of droop can be excessive when the throttle is retarded. The potentiometers 533 and 633b are provided for the purpose of compensating for this droop.

When the pilot finds it necessary to retard the throttles I6 and 16h of No. 2 and No. 3 engines, the sliders 539 and 539D are automatically moved to the left. The movement of slider 539 to the left has the same effect as the movement of the pressure responsive potentiometer slider tdsl to the right, which is the direction in which it is moved upon a drop in induction system pressure. As explained in connection with Figure l, the ultimate effect of this is to cause the waste gate to assume a position closer to fully closed position so as to increase the amount of exhaust gas flowing through the turbine and hence to raise the speed of the turbine and the resultant induction system pressure. Thus. with no change in the position of slider lcd of the controlling potentiometer 9d. the waste `gate is moved to a position tending to increase the induction system pressure. Actually, no increase in the induction system pressure takes place because of the fact that the condition which resulted in the movement of slider 539 to the left, namely the movement of throttle i6 to closed position, also produced a reduction in the amount of exhaust gas so that for a given position of the waste gate, a lower induction system pressure would be maintained. The total voltage across resistor 537 is adjusted by adjusting the position of slider 536. Hence, the compensating eiect of potentiometer 533 is adjusted by potentiometer 532. In initially setting up the apparatus in an airplane, the slider 53S is so adjusted that the compensation introduced by the potentiometer 5325 upon a given throttle movement exactly corresponds to the decreased eifect in the exhaust gases resulting from the closing 4of the throttle. Thus, as the throttle IS is moved towards closed position, the induction system pressure at any given waste gate position remains unchanged, despite the reduction in the amount of exhaust gas.

The network 53th has exactly the same effect upon the control system for engine No. 3 as network 539. t is to be noted that the two networks are connected into their control systems in identically the same manner and the potentiometers are moved in the same way upon movements of the respective throttle levers towards closed position. Hence the movement of the throttles for either the No. 2 or the No. 3 engines towards closed position does not in any way affect the induction system pressure that is maintained. The only eiTect that does result is a reduction in the manifold pressure, which is what is desired.

Considering now the control system for the engine No. l, a somewhat different condition exists when the throttle for engine No. 2 is moved to closed position. The amount of exhaust gases owing to the turbine of engine No. 1 remains the same `because engine No, 1 has not been affected. On the other hand, the manifold pressure is excessively high because of the effect of the pressure selector potentiometer l32. While in the case of engines No. 2 and 3, it is impossible to reduce the manifold pressures by reducing the induction systems pressures, this dihculty does not exist in connection with engines No. 1 and 4 since the superchargers of these engines are not employed for cabin supercharging and ventilation purposes. Consequently, the present system provides means operative when the throttle for engine No. 2 is retarded to automatically 32 reduce the induction system pressure of engine No. 1.

Referring particularly to the apparatus as illustrated in the drawing, it will be noted that when the throttle lever is moved to the left, the slider 55% of potentiometer 553 is moved to the right. This produces the same effect as though slider lia of the induction system pressure responsive potentiometer 9de moves to the left as on an increase in induction system pressure. As previously explained, the movement of slider lil-lia to the left causes the waste gate of the associated turbine to move towards open position to decrease the induction system pressure. Hence, the movement of throttle lever 23 to the left automatically resets the turbine control system to reduce the induction system pressure in connection with engine No. 1. This reduction in induction system pressure is, to a certain extent, overcome by the subsequent effect of the induction system pressure responsive potentiometer elle in attempting to again raise this pressure. As previously explained, however, such a proportioning system inherently has a certain droop so that it is impossible for the induction system pressure responsive means to raise the pressure to the value previously assumed.

The potentiometer 552 is employed to determine the eiiec-t of potentiometer 553. The further the slider 556 is moved to the right, the greater will be the voltage that is impressed across resistor 551 and hence the greater will be the eiiect of potentiometer 553. By a proper selection of the position of slider 55S when the system is installed in an airplane, it is possible for the potentiometer 553 to exert Such an effect that the manifold pressure is automatically reduced to the value desirable from the standpoint of engine operation.

The network 556e functions in connection with engine No. 4 in a manner identical to that described in connection with network 55. In other words, movement of the throttle lever 23h to the left to close throttle I Bb causes a movement of slider 559e to the right to lower the setting of the induction system pressure means.

It will be seen from the above that with the system of Figure 3, it is possible to maintain the desired cabin pressures by use of the turbo superchargers for engines No. 2 and No. 3 while in no way affecting the engine performance adversely. Asy soon as the manifold pressure begins to rise above a value desirable for engine performance, the pilot retards the throttles for the No. 2 and No. 3 engines. rihis does two things. In the Iirst place, it reduces the manifold pressure without reducing the induction system pressure, such as would otherwise happen if the compensating potentiometers 533 and 53322 were not employed. At the same time, the induction system pressure in connection with engines No. 1 and No. 4 is automatically lowered so as to in turn lower the manifold pressure. Thus, the manifold pressure is simultaneously reduced, in connection with all four engines, the reduction being effected in each case in a manner most desirable `from the standpoint of maintaining both the proper cabin pressure and the proper manifold pressure.

Conclusion It will be seen that I have provided an improved pressure control system for an aircraft whereby the same superchargers are employed for both supplying compressed air to the engines and also to the cabins. It will further be seen that I have accomplished this Without in any way interfering with either the maintenance of the desired pressure Within the cabin or the desired manifold pressure for the engines.

While I have shown certain specific embodiments of my invention, it is to be understood that this is for purpose of illustration and that my invention is to be limited only by the scope of the appended claims. I

I claim as my invention:

1. In an aircraft propelled by an internal combustion engine and having a cabin adapted to be maintained at a pressure normally higher than that `of the surrounding atmosphere, means including a compressor for supplying compressed air to said engine, a motor driven by the exhaust gas from said engine for operating said compressor, means for conducting a portion cf the air compressed by said compressor to said cabin to increase the pressure therein, means for controlling Ysaid motor normally to maintain the output of said compressor at a value sufficient to supply the necessary air to said engine, and limiting means for preventing said last named means maintaining the output of said compressor belovv' a value sufficient to supply the necessary air to said cabin.

2. In an aircraft propelled by an internal coinbustion engine and having a cabin adapted to be maintained at a pressure normally higher than that of the surrounding atmosphere, a carburetor for supplying a mixture oi' fuel and air to said engine, means including a compressor for supplying compressed air to said carburetor, means for conducting a portion of the air compressed by said compressor to said cabin to increase the pressure therein, controlling means including a rst device responsive to a condition aecting the pressure of the fuel mixture supplied to said engine for controlling said compressor, means for adjusting said controlling means so as to tend to cause thev same to maintain said condition at a value desirable from the standpoint o engine operation, and means including a second device responsive to a condition aecting the need for air in said cabin for modifying the action of said controlling means so as to tend to cause the air pressure produced by said compressor to be suicient to maintain the desired pressure condition Within the cabin regardless of the setting of said adjusting means.

3. In an aircraft propelled by an internal combustion engine and having a cabin adapted to be maintained at a pressure normally higher than that of the surrounding atmosphere, a carburetor for supplying a mixture of fuel and air to said engine, means including a compressor for supplying compressed air to said carburetor, means for conducting a portion of the air compressed by said .compressor to said cabin to increase the pressure therein, controlling means including a first device responsive to a condition aecting the pressure of the fuel mixture supplied to said engine for controlling said compressor, means for adjusting said controlling means so as to tend to cause the same to maintain said condition at a value desirable from the standpoint of engine operation, and means including a second device responsive to atmospheric pressure for modifying the action of said controllingmeans so as to tend t-o cause the air pressure produced by said compressor to be suicient to maintain the desired pressure condition Within the cabin regardless of the setting of, said adjusting means.

4. Inan aircraft propelled by an internal combustion engine and having a cabin adapted to be 34 maintained atv a pressMW normally higher than that of the surrounding atmosphere, a carburetor for supplying a mixture of fuel and air to said engine, means including a compressor for supplying compressed air to said carburetor, means for conducting a portion of the air compressed by said compressor to said cabin to increase the pressure therein, controlling means including a first device responsive to a condition affecting the pressure oi' the fuel mixture supplied to said engine for controlling said compressor, means for adjusting said controlling means so as to tend: to cause the same to maintain said condition at a value desirable from the standpoint of engine operation, and means including a second device responsive to cabin pre ior modifying the action of said controlling means so as to tend to cause the air pressure produced by said compressor to be sufiicient maintain the desired cabin pressure regardless of the setting of said adjusting means.

5. In. an aircraft propelled by an internal combustion engine and having a cabin adapted to be maintained at a pressure normally higher than that of the surrounding atmosphere, a carburetor for supplying a mixture of fuel and air to said engine, means including a compressor for supplying compressed air to said carburetor and said for conducting a portion of the air compressed by said compressor to said cabin to increase the pressure therein, means for automatically controlling the oW of air through said cabin to maintain a desired pressure therein, controlling means including a first device responsive to a condition afecting the pressure of the fuel mixture supplied to said. engine for controlling said compressor, means for adjusting said compressor controlling means so as to tend to cause the same to maintain said condition at a value desirable from the standpoint of engine operation, and means including a second device responsive to cabin pressure for modifying the action of said compressor controlling means so as to tend to cause the air pressure produced by said compressor to be sufficient to enable said cabin air flow controlling means to maintain the desired cabin pressure regardless oi the setting of said adjusting means.

6. In an aircraft propelled by an internal combustion engine and having a cabin maintained at a pressure normally higher than that of the surrounding atmosphere, a carburetor for supplying a mixture of fuel and air to said engine, a compressor for supplying compressedV air to said carburetor and to said cabin, means responsive to the pressure of the air supplied by said compressor for controlling the operation of said compressor, means responsive to a condition affecting cabin pressure for modifying the action of said previously named means to insure that the pressure of the air supplied by said compressor is adequate to maintain the-proper cabin pressure, a throttle for controlling the flow of the fuel mixture to said engine, and means positioned by said throttle and effective when said` throttle is moved towards closed position to cause an increase in the compressing effect of said compressor.

'7. In an aircraft propelled by an internal combustion engine and having a4 cabin adapted to be maintained at a pressure normally higher than that ofthe surrounding atmosphere, a carburetor for supplying a mixture of fuel and air to said engine, means including a compressor forsupplying compressed air to` said carburetorl and said cabin, a throttle for controlling the iioyv4 of the fuel mixture to said engine, means aiected. by the operation of said engine for driving said corn- 

